Nov 15, 2019 | Allgemein, Topstory

Thin-film photodetectors

Si-based CMOS imagers to detect SWIR wavelengths above 1µm

Thanks to a cross-departmental effort involving material development, semiconductor processing skills, system-level design and much more, imec has realized breakthroughs in the capabilities of silicon-based CMOS imagers to detect short-wave infrared (SWIR) wavelengths above one micrometer.

Figure 1 | By processing a thin-film multilayer stack with photoactive layer sensitive in the infrared range (r.), on top of a Silicon readout circuitry (ROIC), Imec creates an IR-sensitive CMOS imager (l.) that’s compatible with mass manufacturing. (Image: Imec)

Such wavelengths (e.g. the 1450nm and 1550nm bands) are important for the development of applications such as computer vision in mobile devices. Yet, due to optical limitations, these wavelengths are normally invisible to Si-based devices. Conventional approaches that use III-V materials (e.g. InGaAs) can overcome this detection barrier, but are not available at an acceptable price point for consumer devices. Thanks to thin-film photodetector (TFPD) technology, Imec has now developed an end-to-end solution that enables Si-based infrared CMOS sensors at the price level of conventional CMOS imagers. TFPD are multilayer stacks of a few hundred nanometers overall thickness with one of the layers being sensitive to IR. By post-processing these onto a Si-CMOS readout circuit, Imec combines the best of both worlds: infrared detection via a CMOS-compatible process flow. Regarding materials suitable for this thin film, the company is investigating multiple options, ranging from polymer- and small-molecule organic materials to inorganic colloidal quantum-dot layers. The latter, for the moment, is most promising because of the tunable and low-energy bandgap inherent to quantum dots. So far, Imec has built most of its prototypes and demonstrators with PbS quantum-dot materials. The quantities of lead being used remain well within the legal restrictions of the EU ROHS guidelines and other regulations, making them suitable for production. Completely lead-free alternatives are on the roadmap and being investigated as well.

Figure 2 | Schematic illustration of the design choices targeting Imec’s progressive application roadmap. Left: basic IR-detector; Middle: IR detection integrated in visible-light imager; Right: multispectral IR detection thanks to tunable TFPD layers. (Image: Imec)

Progressive application roadmap

In a first instance, monochrome infrared imagers based on a single TFPD stack are created and integrated as a separate die/functionality on the system level. This first implementation is the simplest, as it uses a plain, unpatterned layer of the thin-film photodetector stack. In this scenario, all pixels have the same absorption spectrum, unless you use specific filters. Potential application could be wavelength extension of the face scanner in smartphone cameras, allowing to move to the 1450nm spectrum without adding too much cost or complexity at the system level. Especially for Augmented Reality, this could become a valuable option to enable room-size scanning and applications.

In a second implementation, Imec targets monolithically integrated TFPD stacks into the RGB pixel composition on the CMOS imager itself. In this design, an infrared subpixel can be added next to the conventional red, green and blue photodiodes. This means a separate sensor for IR detection would not be required, reducing both the system footprint and power consumption. Also, it would add an additional layer of information to visible cameras. Think for example about very accessible cameras with depth sensing capability.

The third implementation builds further on the monolithic pixelated design concept, combining multiple TFPD stacks with different active materials. Such configuration would enable pixel-level multispectral sensors in NIR and SWIR ranges, at a very compact form factor and a price point in the range of silicon image sensors. Application potential could be in autonomous vehicles needing long-range scanning capabilities (enabled by 1450nm-sensitive TFPD) as well as visibility in bad weather or low-light conditions (enabled by 1550nm-sensitive TFPD). Another application example could be in material sorting applications, where tuning pixels to characteristic wavelengths would add material-determination capabilities (e.g. discrimination of vegetation vs. buildings or real vs. fake plants).

For the first concept – the monochromatic IR sensor – Imec has built a complete end-to-end prototype integrated into a camera. Starting with a 200mm ROIC wafer processing in the foundry. Post-processing and TFPD integration (on die or wafer level) was executed in the Imec fab, as well as chip packaging and buildup of the camera module. For the two monolithic designs, who are still in an earlier stage of research, the ambitions and roadmap are similar.

Author: Ph.D. Paweł E. Malinowski, Program Manager User Interfaces & Imagers; Ph.D. David Cheyns, Team Leader Future Interactive Thin-Film Technology; Ph.D. Pierre Boulenc, Leader Pixel Devices Team, Imec
www.imec-int.com

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